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DP83848YB: Drive Strength/Max. Sink Current of the MDIO Pin

Utku Karakas
Prodigy 70 points
Part Number: DP83848YB
Other Parts Discussed in Thread: LSF0102-Q1

Tool/software:

Hello TI Team,

I'm using DP83848YB as an ethernet debug interface in an automotive project. DP83848YB was until now interfaced directly to a MCU that had 3.3V pads for MDC (push-pull) / MDIO (open drain). MDIO was connected to 3V3 per 1k5 pullup resistor as specified in the DP83848YB datasheet. The MDC clock is set to 2,48 MHz in our MCU.

The new MCU that I'm using supports only 5V pads for MDC/MDIO, so I'm going to have to use a level translator (I'm considering LSF0102-Q1) for these 2 signals. LSF Translators need strong pull-up resistors for high data rates, especially for bidirectional open drain signals such as MDIO.

I'm trying to calculate an appropriate pull-up resistor for the MDIO line, which will satisfy MDIO specs and also doesn't exceed abs max current sink ratings of the MCU and DP83848YB. 

What is the max. sink current for the MDIO pin of DP83848YB? In the datasheet there is no abs max rating. It only specifices VOL_max = 0,4 V at IOL = 4 mA under DC specs. 

Thanks,

Utku

  • 0
    TI__Mastermind 39795 points

    Hello Utku,

    DP83848 is an older device with support limited to datasheet. Regarding MDIO configuration, typically we see a 1.5k-2.2k for MDIO work plenty fine. This accounts for the internal PU that is already existing within PHY's MDIO pin.

    Sincerely,

    Gerome

  • 0 in reply to Gerome Cacho
    Prodigy 70 points

    Hello Gerome,

    thanks for the feedback. I think that the situation is a bit more complicated due to LSF1012-Q1 that I'm going to use between the CPU and the DP83848. I'll have to implement an additional pull-up resistor to the 5V side (CPU) as required per LSF1012-Q1 datasheet. The 1k5 pull-up on the 3.3V side (DP83848) will stay the same.

    The additional pull-up on the 5V side will increase the sink current: I_OL = (3.3V/1.5k) + (5V/R_pull_new). Besides, I can't just choose R_pull_new e.g. 10k, because on the 5V side of LSF1012-Q1, the max data rate depends on the R_pulll_new and paraistic pin capacitances.

    BR, 

    Utku

  • +1 in reply to Utku Karakas
    TI__Mastermind 39795 points

    Hello,

    I see. Typically, our VOH/L specifications are assuming 2mA sink/source, which is fairly large for an IO. 

    Perhaps if there is an EVM for the LSF, you can pair up with our DP83848 EVM to evaluate what PU may be needed.

    Sincerely,
    Gerome

  • +1 in reply to Gerome Cacho
    Prodigy 70 points

    Hello Gerome,

    I might try that. Since DP83848 specifies VOL_max = 0,4 V at IOL = 4 mA, I'm at the moment gonna assume that this is the max current it can sink and will design the circuit accordingly. 

    BR,

    Utku